One and two dimensional symbologies have been proposed for optically recording machine readable digital information on plain paper documents and the like. The principal purpose of these symbologies is to establish reliable digital communication channels in the image domain. While 1-D symbologies still have an important role, there has been a shift toward 2-D symbologies because they tend to more fully respond to the demand for broader band width communication channels and to the related need for increased spatial bit density recording capacities.
The Xerox Dataglyph.TM. symbology is an especially promising 2-D symbology for optically recording machine readable digital information on traditional and not so traditional image recording media. Its spatial bit density recording capacity and its tolerance to image distortion and image degradation compare favorably with the best of the other known 2-D symbologies. Furthermore, the Dataglyph.TM. symbology is esthetically far superior to these other 2-D symbologies because of its substantially homogeneous visual appearance. Indeed, when used to record at higher spatial bit densities, the Dataglyph.TM. symbology has a unobtrusive, generally uniform grayscale visual appearance. This esthetic advantage is an important attribute, especially for applications that involve combining machine readable and human readable information on documents which are intended to favorably impress human readers.
Dataglyphs.TM. are based on relatively simple "self-clocking glyphs codes." In accordance with the Dataglyph.TM. symbology, such a code is composed of elongated slash-like symbols or glyphs which are written on a regular lattice of centers at tilt angles of approximately +45.degree. and -45.degree. from vertical for the recording of binary "0's" and "1's," respectively, No information is encoded in the spaces between the glyphs or in the relative spacing of the transitions between the glyphs and the background. Instead, each bit is independently and explicitly encoded by a glyph. The glyphs, therefore, provide a reliable clock for reading the bits out of the code, even in the presence of significant image distortion and/or image degradation (i.e., the bits are recoverable if the glyphs can be located and identified with sufficient precision to enable any errors to be corrected by the error correction protection, if any, that is provided).
Higher spatial bit density Dataglyphs.TM. conventionally are written on regular rectangular lattice-like patterns of centers by printers operating at printer resolutions on the order of 300-600 d.p.i. (dots/inch). In practice, the individual glyphs may be as small as 3-5 pixels long, and these glyphs may be written at a nominal center-to-center spacing of only about 5-7 pixels horizontally and vertically. The high spatial density at which the glyphs are written causes the recorded code to have a generally uniform grayscale appearance because the individual glyphs tend to blend together when viewed by the unaided human eye at normal reading distance under normal lighting conditions. However, there is a problem with the inter-glyph interference which can manifest itself in these higher density glyph codes.
Inter-glyph interference occurs when the ends of diagonally adjacent glyphs come into contact or near contact (i.e., such close proximity that the gap between them cannot be resolved by the scanner that is used to read the code). Such interference hinders the reading of the code by making it more difficult to accurately locate the centers of the individual glyphs and to unambiguously determine the values encoded by them. Thus, interglyph interference is a limiting factor on the density at which the glyphs can be recorded, scanned and read. It, therefore, would be beneficial to reduce the risk of interglyph interference, especially if this can be accomplished without sacrificing bit recording density.